1. Bone Development and Growth

2. The process by which bone forms.
Ossification
The process by which bone forms.

3. Bone Development Begins in first two months of prenatal life.
2 methods – 1. Intramembranous ossification, formation directly on or within fibrous connective tissue. 2. Endochondral ossification, formation of bone within cartilage.
Both methods lead to the same structure in mature bone
Different methods of development in which both replace preexisting connective tissue with bone.

4. Intramembranous Ossification
Flat bones of the skull, the mandible and clavicles form this way.
1. Cells in the mesenchyme ( tissue that all other connective tissue arise), come together and form precursor cells (osteoprogenitor) transform into osteoblasts, capable of secreting collogen and mineral salts.

5. Intramembranous
2. Osteoblasts cluster and become active by secreting new organic matrix around themselves.
3. After secretion stops the cells now called osteocytes are located in the lacunae, extend narrow cytoplasmic processes into canaliculi.
4. Within a few days mineral salts have been deposited and the matrix hardens.

6. Intramembranous
Bone matrix develops into trabeculae
Which fuse to form spongy bone
The spaces between the trabeculae fill with red marrow
The mesenchyme develops into periostem
Eventually most spongy bone is replaced by compact bone except at the center of the bone.

7. Endochondral Ossification
Bones that form from cartilage.
All other bones
Begins around 6th week of life
At site where bone will be, cells come together to shape the future bone.
Cells develop into cartilage producing cells (chondroblasts) that change into hyaline cartilage. (cartilage model)

8. Endochondral 3.The cartilage models grows and thickens
4. Blood vessels penetrate it, causing some chondroblasts to die while others transform into osteoblasts.
5. Death of chondroblasts leaves spaces in the bone, in long bone this happen in the center of the diaphysis, called the primary ossification center.
6. Osteoblasts migrate to the spaces and produce spongy bone. At the same time periosteum cells deposite a thin layer of compact bone, on the surface of the model (cartilage).

9. Endochondral
7. Now more blood vessels penetrate the model at the epiphysis.
8. In a long bone this is known as the Secondary ossification center.
9. The conversion of the cartilage to bone happens again, in the epiphysis.

10. Endochondral
10. The space formed by the cartilage destruction, expands to form a medullary cavity.
11. The epiphysis fills with spongy bone with a thin layer of compact bone surrounding, the walls of the diaphysis thickens.
12. A thin layer of cartilage remain between the epiphysis and the diaphysis, called the epiphyseal plate

11. Animations of both forms of ossification

12. Bone Growth- Interstitial
Lengthwise (interstitial) growth- epiphyseal plate
Cartilage region that contains chondroblasts that produce new cells and intercellular material, pushed toward epiphysis.
At the other end of the plate the cartilage in regenerating itself.
Bone will continue to grow as long as the chondroblasts are active. ( regulated by hormones)
Epiphseal line is a permanent fuse of the epiphysis and the diaphysis

13. Bone Growth- Appositional
Width growth of a long bone
Osteoblasts in the periosteum, deposite new compact bone along the outer surface of the diaphysis.
Medullary cavity expands by destruction of the bone along the inner surfaces, performed by osteoclasts (dissolve bone matrix, be secreting enzymes)
Unequal process, more bone deposited then removed, making a thicker and stronger bone.

14. Bone remodeling Bone does not become inactive once it is done growing.
Osteoblasts deposit new bone, osteoclasts reabsorb old bone.
The process is known as bone remodeling.
Not all bones undergo it, but bones and sections of bones that undergo the most stress or injuries undergo it more frequently.